Hydrolyzed starch polyacrylonitrile graft copolymers which exhibit the capacity to absorb from about 300 to 1000 times their weight of deionized water are known at this time. The development of these compositions was carried out by the Northern Regional Research Laboratory, Peoria, Ill. The hydrolyzed starch polyacrylonitrile graft copolymer is produced by the free radical method of polymerization. In this method, starch, either gelatinized or ungelatinized, is exposed to a catalyst such as ceric ammonium nitrate which acts as a catalyst to generate free radicals in the starch chain. These free radicals can also be produced by radiation. Polyacrylonitrile chains become attached to these free radicals by copolymerization. A wide range of substitution in these copolymers is known in the art. For example, U.S. Pat. No. 3,035,099 shows the preparation of copolymers in which the starch to polyacrylonitriles molar ratios range from 1:1.5 to 1:9. The variations in molar ratio of the components of the copolymer is not critical to the practice of this invention. The resulting material is then saponified in sodium hydroxide to hydrolyze the polyacrylonitrile chains to carboxamide and alkali metal carboxylate groups mixed with metal salts. Drying the hydrolyzed material can be accomplished by tumble air drying or vacuum drying. After drying, the material can absorb about 300 to 400 times its weight of deionized water. Washing the absorbent polymer before drying with alcohol increases its absorbency to 800 to 1000 times its weight of deionized water.
The absorbent polymer can be made as film, flakes, powder or mat. These forms take up water, swelling but not dissolving and hold it in expanded duplications of their own dry shapes. Films extend and thicken in all dimensions. Powders become piles of water textured like crushed ice. A flake expands to a clear, angular piece of water. The swollen forms shrink in dilute acid, expand again in dilute alkali solution. They also shrink as they dry and expand again when absorbing water.
The absorbent polymer, with these properties, can be mixed with or coated on a wide variety of materials including, for example, sand, straw, sawdust, seeds and roots, natural or synthetic fibers, flour, gelatin and starch. It can hold water in soils, animal bedding and kitty litter, toweling and diapers, bandages, surgical pads and dental absorbents.
On addition to aqueous fluids, the absorbent polymer swells rapidly, absorbing the aqueous fluid in the process. The rate of swelling (hydration rate) and the amount of fluid capable of being absorbed varies with the dissolved ion content of the fluid; however, the hydration rate and volume absorbed are significant even with relatively high ion content fluids.
While rapid hydration rates and a high degree of absorbency of the absorbent polymer are desirable in many applications, the rapid rate of hydration can be a disadvantage when aqueous fluid is added to the dry, hydrolyzed starch polyacrylonitrile graft copolymer powder. In this case, the surface of the quantity of absorbent polymer powder in contact with aqueous fluid swells and absorbs the fluid so quickly that the resulting swollen gel reduces or blocks further penetration of the aqueous fluid. This, in turn, reduces or retards full swelling or absorbency of the product. In some cases wet-out of the dry product can be improved by increasing granulation of the product, but large granular material is not always feasible and even when it is, the improvement is only partial. Thus, a need exists for a highly absorbent product which wets-out quickly and completely when exposed to an aqueous fluid, but which still retains acceptable absorbency characteristics.
Applicants are aware of the teachings of the Reid Pat. No. 4,051,086 regarding the use of a difunctional dialdehyde, glyoxal, as a crosslinking agent in the modification of a crosslinked grafted polysaccharide to improve the wicking characteristics of an absorbent polymer. As further taught in the Reid patent, however, while glyoxal appears to be effective for the materials contemplated therein, it does not appear to be effective in improving the wicking of other crosslinked absorbent derivatives. In fact, it does not appear to be effective with hydrolyzed starch polyacrylonitrile graft copolymer, the absorbent polymer contemplated by the present invention.